It is often desirable to detect particles in fluids and determine their size distribution. Microscopes and cameras may be used to accomplish these measurements if the liquid is transparent, and the liquid can be viewed by the measurement devices. Light scattering may also be used to detect particles, but liquid transparency to the light is still a requirement. Various optical methods that use coherent light (lasers, for example) as well as incoherent light in the optical region of the electromagnetic spectrum have been reported to be useful for particle measurements in transparent or translucent fluids and in situations where particle concentrations are low to moderate. In situations where detection of the presence of particulate matter in a liquid flowing through a transparent or opaque pipe where the liquid is optically opaque, such as crude oil, conventional techniques do not work.
Additionally, it is of interest to monitor fluid flow and fluid flow patterns. This may also be achieved by visual observation, but again the liquid being studied must be optically transparent.
Ultrasonic imaging is used extensively in nondestructive testing for detecting cracks and other defects. It is also used for medical imaging to visualize various internal organs in humans or animals.